During the drilling of oil and gas wells, drilling fluid is circulated through the interior of the drill string and then back up to the surface through the annulus between the drill string and the wall of the borehole. The drilling fluid serves various purposes including lubricating the drill bit and pipe, carrying cuttings from the bottom of the well borehole to the rig surface, and imposing a hydrostatic head on the formation being drilled to prevent the escape of oil, gas, or water into the well borehole during drilling operations.
There are numerous possible causes for the drill string to become stuck during drilling. Differential sticking, one of the causes for stuck pipe incidents, usually occurs when drilling permeable formations where borehole pressures are greater than formation pressures. Under those conditions, when the drill pipe remains at rest against the wall of the borehole for enough time, mud filter cake builds up around the pipe. The pressure exerted by drilling fluid will then hold the pipe against the cake wall.
Some warning signs that put one on notice of the possibility of differential sticking are the presence of prognosed low pressure along with depleted sands; long, unstabilized bottom-hole assembly (hereafter BHA) sections in a deviated hole; loss of fluid loss control and increased sand content; and increasing overpull, slack off or torque to start string movement.
Indications of the actual presence of differential sticking include a period of no string movement; the string cannot be rotated or moved, but circulation is unrestricted.
Methods of freeing differentially stuck drill string include applying torque and jar down with maximum torque load; using a spot pipe releasing pill if jarring is unsuccessful; and lowering mud weight, which may have implications with respect to hole stability. The overpull required to release the pipe may exceed rig capacity, and even cause collapse of the rig. It would be very beneficial if a method were available to reduce the required freeing force so that the existing rig would be adequate for overpull without possibly causing collapse.
Application of wave energy in the oil industry is known, however the most common application of ultrasonic energy is cleaning of electronic microchips in the semiconductor industry and daily household cleaning of jewelry.
In addition to the use of acoustic and ultrasonic methods for core measurements in the laboratory, logging, and seismic applications in the field, acoustic energy has been shown by Tutuncu and Sharma to reduce the lift-off pressure of mud filter cakes by a factor of five. See Tutuncu A. N. and Sharma M. M., 1994, “Mechanisms of Colloidal Detachment in a Sonic Field”, 1st AIChE International Particle Technology Forum, Paper No 63e, 24-29.
Other uses of ultrasonic energy include supplying the energy through downhole tools into hydrocarbons to facilitate the extraction of the oil from the well by reducing the viscosity of the oil. See, for example, U.S. Pat. Nos. 5,109,922 and 5,344,532. U.S. Pat. No. 5,727,628 discloses the use of ultrasonic to clean water wells.
Freeing pipe using vibrational energy has also been tried in recent years. U.S. Pat. No. 4,913,234 discloses a system for providing vibrational energy to effect the freeing of a section of well pipe which comprises: a) an orbital oscillator including a housing; b) an elongated screw shaped stator mounted in said housing and an elongated screw shaped rotor mounted for precessionally rolling rotation freely in said stator; c) means for suspending said oscillator for rotation within said drill pipe about the longitudinal axis of the drill pipe in close proximity to the stuck portion thereof; and d) drive means for rotatably driving said rotor to effect orbital lateral sonic vibration of said housing such that said housing precesses laterally around the inner wall of said pipe, thereby generating lateral quadrature vibrational forces in said pipe to effect the freeing thereof from said well bore.
U.S. Pat. No. 5,234,056 discloses a method for freeing a drill string which comprises a) resiliently suspending a mechanical oscillator from a support structure on an elastomeric support having a linear constant spring rate; b) coupling said oscillator to the top end of the drill string, the elastomeric support creating a low impedance condition for vibratory energy at said drill string top end; c) driving said oscillator to generate high level sonic vibratory energy in a longitudinal vibration mode so as to effect high longitudinal vibratory displacement of the top end of the drill string; and d) the drill string acting as an acoustic lever which translates the high vibrational displacement at the top end of the drill string into a high vibrational force at the point where the drill string is stuck in the bore hole, thereby facilitating the freeing of the drill string.
Often when a drill pipe is differentially stuck the result is that it has to be cut and the target zone cannot be reached by the optimal route. It would be extremely desirable in the art if a method were available which provided a means of reducing the amount of force required for freeing a stuck drill pipe. Such a method could potentially save enormous amounts of time and money in drilling operations.
In the present invention, it has been discovered that the auxiliary use of ultrasonic energy can help reduce the pipe contact area, thus reducing the required freeing force and often permitting the existing rig to be sufficient for use in the overpull. The present invention will save rig time and prevent sidetracking of the well, a high cost operation especially in offshore deepwater environments.